The presence of large deposits of oil shale in the Rocky Mountain region of the United States has given rise to extensive efforts to develop methods of recovering shale oil from kerogen in the oil shale deposits. The term "oil shale" as used in the industry is, in fact, a misnomer; it is neither shale nor does it contain oil. It is a formation comprising marlstone deposit containing an organic material called "kerogen" which, upon heating, decomposes to produce liquid and gaseous products. It is the formation containing kerogen that is called "oil shale" herein and the carbonaceous liquid product is called "shale oil".
The recovery of liquid and gaseous products from oil shale deposits has been described in several patents, one of which is U.S. Pat. No. 3,661,423, issued May 9, 1972, to Donald E. Garrett, assigned to the assignee of this application, and incorporated herein by this reference. This patent describes the formation of a fragmented permeable mass of oil shale particles in a subterranean formation containing oil shale by undercutting a portion of the subterranean formation, leaving unfragmented formation supported by a plurality of pillars. The pillars are removed, e.g., with explosive, and the unfragmented deposit is expanded to provide a permeable mass of formation particles containing oil shale, referred to herein as an in situ oil shale retort. Hot retorting gases are passed through the in situ oil shale retort to convert kerogen contained in the oil shale to liquid and gaseous products.
One method of supplying hot retorting gases used for converting kerogen contained in the oil shale, as described in U.S. Pat. No. 3,661,423, includes establishment of a combustion zone in the retort and introduction of an oxygen-supplying combustion zone feed into the retort on the trailing side of the combustion zone to advance the combustion zone through the fragmented mass. In the combustion zone, oxygen in the gaseous feed mixture is depleted by reaction with hot carbonaceous materials to produce heat and combustion gas. By the continued introduction of the oxygen-supplying feed into the combustion zone, the combustion zone is advanced through the fragmented mass. The effluent gas from the combustion zone passes through the retort on the advancing side of the combustion zone to heat the oil shale in a retorting zone to a temperature sufficient to produce kerogen decomposition called "retorting". Such decomposition in the oil shale produces gaseous and liquid products, including gaseous and liquid hydrocarbon products and a residual carbonaceous material. The resulting liquid and gaseous products pass to the bottom of the retort for collection.
It is desirable that the retort contain a reasonably uniform fragmented permeable mass of formation particles having a reasonably uniformly distributed void fraction so gases can flow uniformly through the retort, resulting in maximum conversion of kerogen to shale oil. A uniformly distributed void fraction in the direction perpendicular to the direction of advancement of the combustion zone is important to avoid channeling of gas flow in the retort. In preparation for the described retorting process, it is important that the formation be fragmented and displaced, rather than simply fractured, in order to create high permeability; otherwise, too much pressure differential is required to pass gas through the retort.
It has been proposed that oil shale be prepared for in situ recovery by first undercutting a portion of the formation to remove from about 5% to about 25% of the total volume of the in situ retort being formed, leaving the unfragmented portion supported by pillars. The pillars are then explosively expanded and, after a time delay, the unfragmented formation is expanded by detonating explosive placed in the pillars and in the unfragmented formation. This explosive expansion of pillars and unfragmented formation fills the void created by the undercut with a fragmented permeable mass of particles.
The general art of blasting rock formations is discussed in The Blasters' Handbook, 15th Edition, published by E. I. duPont de Nemours & Company, Wilmington, Delaware.
Other methods for preparing formation for in situ recovery are described in U.S. Pat. Nos. 4,043,597 and 4,043,598, both assigned to the assignee of this invention and both incorporated herein by this reference. According to these patents, at least two voids vertically spaced apart from each other are excavated in the subterranean formation. This leaves a zone of unfragmented formation between adjacent voids. Explosive is placed in blasting holes and detonated to expand formation in the intervening zone toward both voids.
U.S. Pat. No. 4,146,272 describes a method for forming an in situ oil shale retort by expanding formation toward vertically spaced apart voids containing support pillars. The pillars are explosively expanded to spread the particles thereof uniformly across the void, and unfragmented formation adjacent the void is explosively expanded toward the void before overlying, unsupported formation can cave into the void. U.S. Pat. No. 4,146,272 is incorporated herein by this reference.
To insure uniform void fraction distribution, pillars are explosively expanded first and then, after a time delay, the remaining unfragmented formation is explosively expanded, either in a single explosion or in a further series of explosions in a single round. One problem caused by use of a time delay method of blasting is that ground movement and/or airborne rock fragments ejected from a previous explosion can sever explosive initiating means. The initiating means, for example, can be trunk lines containing tie-up systems of detonating cord and time delay devices. Severing a trunk line can result in cutoff of a blasthole or blastholes serviced by the severed trunk line where the explosive in the blasthole is not initiated due to the severance. The lack of initiation of explosive in the blastholes causes formation in the area to remain unfragmented, resulting in an uneven distribution of void fraction of fragmented permeable mass in the in situ retort. In order to substantially decrease the probability of having a cutoff blasthole, it is, therefore, desirable to initiate all of the explosive trains downhole prior to the first explosions in a round of time delayed explosions. Explosive trains include initiating devices such as detonating cord, time delays, and primers.
In order to initiate the explosive trains in the blastholes prior to the first explosive expansion in a round of time delayed explosions, time delays must be available having both a required delay period and a required accuracy of timing. Time delays are presently commercially available with the desired delay period up to a delay period of about 200 milliseconds. Such time delays are generally available with delay periods of 25 milliseconds, 50 milliseconds, and continuing in 25 millisecond increments up to a delay of about 200 milliseconds. It is often required that time delays which are longer than about 200 milliseconds be provided for explosive expansion of unfragmented formation while forming an in situ oil shale retort. The presently available time delays which are longer than about 200 milliseconds do not provide a time delay in the required 25 millisecond increments and, in addition, may not have the required accuracy of delay. Both accuracy of timing and having the proper time interval between explosive expansions are significant while forming an in situ retort so that blastholes are detonated in the proper sequence. Explosive expansions having the proper time interval result in the creation of an in situ retort with a uniformly distributed fragmented permeable mass.
There is a need to provide a sufficiently accurate time delay of over 200 milliseconds in 25 millisecond intervals so as to enable total accurate time delays of 225 milliseconds, 250 milliseconds, 275 milliseconds, and so forth, up to a maximum required time delay in explosive expansions, for formation of in situ oil shale retorts.
The provision of sufficiently accurate time delays of over 200 milliseconds in 25 millisecond time increments allows initiation of all explosive trains downhole simultaneously, thereby substantially eliminating cutoff blastholes.